I want to rewrite subdomains and folders to params - .htaccess

Question

I like to rewrite my subdomain site (wildcard) and when exists folders to params for the index.php file.
So
www.domain.com
www.domain.com/js/file.js
www.domain.com/css/file.css
www.domain.com/images/file.jpg
should be preserved!
And these should change to parameters for index.php
sub.domain.com/
to
www.domain.com/index.php?key1=sub
sub.domain.com/2015
to
www.domain.com/index.php?key1=sub&key2=2015
sub.domain.com/2015/12
to
www.domain.com/index.php?key1=sub&key2=2015&key3=12
Is this possible?
What I have sofar is
RewriteCond %{HTTP_HOST} !^www\.domain\.com [NC]
RewriteCond %{HTTP_HOST} ^([^.]+)\.domain\.com [NC]
RewriteRule ^/?(.*)$ $1?page=%1 [QSA,L,NE]
Thank you very much!
So I rewritten some of #anubhava his htaccess to this
Options +FollowSymLinks
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} -s [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTP_HOST} ^([^.]+)\.gebruikmaar\.nl [NC]
RewriteRule ^/?(.*)$ $1?page=%1 [QSA,L]
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTP_HOST} ^([^.]+)\.gebruikmaar\.nl [NC]
RewriteRule ^(.*)([^/][0-9]*)(/?)$ $1?page=%1&key2=$1 [QSA,L]
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTP_HOST} ^([^.]+)\.gebruikmaar\.nl [NC]
RewriteRule ^(.*)([^/][0-9]*)/([^/][0-9]*)(/?)$ $1?page=%1&key2=$1&key3=$2 [QSA,L]
And it almost works!
This url:
http://agenda.gebruikmaar.nl
Becomes
http://www.gebruikmaar.nl/index.php?page=agenda
BUT :-(
This url:
http://agenda.gebruikmaar.nl/2015
Becomes:
http://www.gebruikmaar.nl/index.php?page=agenda&key1=201
So where's the last character (5) at?

Answer 1

You can use this code in your DOCUMENT_ROOT/.htaccess file:
RewriteEngine On
RewriteBase /
# skip all files and directories from rewrite rules below
RewriteCond %{REQUEST_FILENAME} -d [OR]
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [L]
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTP_HOST} ^([^.]+)\.domain\.com [NC]
RewriteRule ^/?$ index.php?key1=%1 [QSA,L]
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTP_HOST} ^([^.]+)\.domain\.com [NC]
RewriteRule ^([^/]+)/?$ index.php?key1=%1&key2=$1 [QSA,L]
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteCond %{HTTP_HOST} ^([^.]+)\.domain\.com [NC]
RewriteRule ^([^/]+)/([^/]+)/?$ index.php?key1=%1&key2=$1&key3=$2 [QSA,L]